The present invention relates to a thermal head for use in recording apparatuses such as facsimile, full color printer, word processor and so on, and more particularly, to improvements in a resistor layer which is one of major components of the thermal head, and the manufacturing method thereof.
The thermal head is mainly composed of at least a pair of electrodes, resistor layers in contact with both the electrodes, base plates for supporting the electrodes and the resistor layers on the surfaces thereof, with at least the surfaces thereof being of insulating property, and abrasion-resistant layers formed on the resistor layers. And there are a thin film type and a thick film type depending upon how to manufacture it. The thin film type is formed by the sputtering, evaporation, etc. of an electrode, a resistor layer, an abrasion-resistant layer in vacuum. Also, the thick film type obtains gold electrodes, a resistor layer composed of a glass layer, RuO.sub.2 being scattered therein, and an abrasion-resistant layer composed of glass, by the respective printing, heating operations of, for example, paste of a decomposable organic compound of the gold, paste containing RuO.sub.2 and glass frit, and paste of borosilicate glass frit, so that the thick film type may provide a thermal head of higher reliability, lower cost than the thin film type.
The thermal head heats the specified region of the resistor layer in contact with both the electrodes through the current flowing between a pair of electrodes so as to heat the specified region of the recording member, for example, a heat sensitive recording paper for giving one dot portion of recording. Accordingly, the important characteristics to be demanded for the thermal head are that the heating of the resistor layer is efficiently transmitted onto the side of the recording paper, and the heating of the resistor layer between the individual electrode pair disposed normally in a line shape is uniform. As the resistances of these resistor layers are unequal, and the respective heating amount is uneven, the concentration of the individual recording dots to be recorded on the recording paper become unequal, thus causing the lines of variable density on the recording to make the recording quality worse. The characteristics are emphasized especially as the thermal head for full color printer use which demands the gradation record. A cause for such uneven record concentration is considered to be the dispersion of the resistance values of the individual resistor dots. In order to reduce the resistance value dispersion of such individual resistor dots, a trimming step, in the thick film method, is adopted. This step applies the overload pulses on the individual dots of the resistor layer, thus making it possible to have the resistance value within .+-.0.5% of the target. On the other hand, in the thin film type, the resistance value of the individual resistor dot may be provided within .+-.2.5% by the controlling operation of the conditions of the evaporation and sputtering for obtaining the resistor. But in the head of the thin film system, it is difficult to further improve the dispersion of the present resistance value, and in the head of the thick film system, the present system has problems as described hereinafter. The resistor layer of the thermal head of the present thick film type is formed by the screen-printing, heating of the paste composed of the resistor component RuO.sub.2, glass frit, organic binder. But, as the paste is a mixture between RuO.sub.2 powder and glass powder, the resistor layer to be produced by the paste is also a mixture of them. And, if RuO.sub.2 powder which is small in granular diameter is used, the resistor layer to be produced by the paste is often aggregated or is worse in the dispersion in the glass matrix, so that the powder becomes very large in diameter in the resistor layer obtained. In the result, the current is adapted to flow through the RuO.sub.2 powder in contact against each other. Accordingly, in order to obtain a resistor having a uniform resistance value, it is necessary to provide a considerable amount of RuO.sub.2 powder. On the other hand, as the preferential change in the resistance value is caused at a portion easy to be trimmed, especially, in a one resistor dot even if the dot resistance value is made constant by the trimming, the heating is to be concentrated in one portion of one dot in the actual recording even when the dot resistance value has reached the target value, so that the normal dot shape is not obtained. The deviation of the current pass in such one resistor dot is due to unequal distribution of the conductive element like the RuO.sub.2 in one resistor dot.
As described hereinabove, in the conventional method of forming the resistor layer from a mixture between the RuO.sub.2 and the glass powder, it was difficult to obtain the resistor layer uniform in the resistor value.